Reversible male sterility in transgenic plants by expression of cytokinin oxidase

ABSTRACT

Methods for the production of reversibly male-sterile plants by introduction of a polynucleotide encoding a cytokinin oxidase are disclosed along with nucleic acid constructs and transformed cells useful in the production of such plants. Also disclosed is the use of plants containing recombinant nucleic acid sequences in preventing pollination of plants with pollen containing one or more transgenes and in the introduction of economically important traits into elite varieties of plants.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/326,184, filed Dec. 20, 2002, now U.S. Pat. No. 7,230,168, which claims the benefit of provisional application Ser. No. 60/343,129, filed Dec. 20, 2001, each of which is hereby incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The current invention generally relates to a method for the production of reversible male sterility in transgenic plants. The method comprises transforming a plant cell with a nucleic acid construct encoding cytokinin oxidase where expression of the cytokinin oxidase inhibits pollen formation or male organ development in the plant. Fertility restoration in the plant is achieved by restoration of normal cytokinin levels.

BACKGROUND

Reversible male sterility is a valuable trait in the production of hybrid seed. Since phytohormones are believed to be involved in the plant male organ development, male sterility can potentially be induced by alteration of endogenous hormone levels. Several attempts have been made to chemically induce male sterility with the application of phytohormones (Moore, Science, 129:1738-1740, 1959; Eeninck et al., Acta Bot. Neerla, 27:199-204, 1978; Sawhney, Can. J. Botany, 61:1258-1265, 1981; Saini et al., Aust. J. Plant Physiol., 9:529-537, 1974; Chandra Sekhar et al., Sex. Plant Reprod., 4:279-283, 1991). Although these exogenous hormone treatments successfully produced male sterile plants, the narrow window of application, the necessity of applying chemicals continuously to produce sterility, lower seed yields on the treated plants after crossing, and occasional lapses in achieving complete sterility make hormonal induction of male sterility impractical for commercial hybrid seed production. The pleiotropic effects of the hormone applications as well as environmental influences on exogenous hormone uptake, translocation and metabolism likely cause these inconsistencies.

The discovery of genes involved in hormone pathways and promoters conferring tissue specific expression, permit control of endogenous phytohormone levels in specific tissues, and allow precise, effective induction of male sterility. More importantly, male sterility induced by phytohormone perturbations can theoretically be reversed by application of the appropriate phytohormonal agonists or antagonists. Fertility restoration is critical as it enables inbred male sterile lines to be maintained.

The use of molecular biology to produce male sterility in plants has been described. In a series of patents, Cigan et al. disclose the use of the anther-specific promoter 5126 and variants thereof to control expression of sequences related to pollen formation (U.S. Pat. Nos: 5,689,049; 5,689,051; 5,763,243; 5,792,853; 5,795,753; 5,837,851; and 6,072,102). Albertson et al. (U.S. Pat. No. 5,962,769) describe a method for producing reversible male-sterile plants by introduction of an expression vector which produces avidin. Male sterility is reversed by crossing to a “restorer” line expressing anti-sense avidin or a suitable ribozyme. Alternatively, sterility can be reversed by application of biotin. Baudot et al. (U.S. Pat. No.: 6,207,883 B1) disclose the male fertility gene Ms41-A in Arabidopsis and a related maize gene Zm41-A. Baudot et al. further disclose that mutation of the Ms41-A gene resulted in male sterility. Michiels et al. (U.S. Pat. No. 6,025,546) utilized a method of transforming plants with a coregulating gene combined with a male sterility gene to generate a higher frequency of male sterile transgenic plants. Poovaiah et al. (U.S. Pat. No. 6,077,991) disclose the suppression of calcium/calmodulin-dependant protein kinase expression by the use of antisense constructs to induce male-sterility. Scott et al. (U.S. Pat. No. 5,955,653) disclose the use of a tapetum-specific callase gene and its promoter to induce male-sterility. Van Tunen et al. (U.S. Pat. No. 6,005,167) disclose a method for inducing male sterility by the use of recombinant polynucleotides that inhibit the expression of one or more gene involved in the synthesis of chalcone or one of its precursors.

SUMMARY OF THE INVENTION

Among the several aspects of the invention is provided a method for producing a plant characterized by reversible male-sterility comprising transforming a plant cell with a construct containing a polynucleotide encoding a cytokinin oxidase and regenerating a plant from the plant cell wherein expression of the cytokinin oxidase inhibits pollen formation or male organ development in the plant and restoration of male-fertility is possible by restoration of normal cytokinin levels. Restoration of normal cytokinin levels can be achieved by application of cytokinin, a cytokinin oxidase inhibitor, or any combination thereof.

In one aspect, the construct further comprises a regulatory sequence and a termination sequence, both operably linked to a polynucleotide encoding the cytokinin oxidase. In another aspect, the regulatory sequence comprises a promoter. Suitable promoters can be selected from promoters that are constitutive, inducible, environmentally-regulated, developmentally-regulated, organelle-specific, cell-specific, tissue-specific, male specific, anther-specific, pollen-specific, stamen-specific, tapetum-specific or any combination thereof, for example, an inducible, male-specific promoter.

In a further aspect, the method further comprises transforming the plant cell containing a polynucleotide encoding a cytokinin oxidase with at least one additional nucleic acid construct comprising an inducible promoter and a transcription termination sequence both operably linked to a polynucleotide encoding a cytokinin biosynthesis enzyme active in the plant. In an alternative aspect, the male sterile plant is produced by sexually crossing a plant comprising a polynucleotide encoding a cytokinin oxidase with a plant, typically of the same variety, comprising at least one nucleic acid construct containing an inducible promoter and a transcription termination sequence both operably linked to a polynucleotide encoding a cytokinin biosynthesis enzyme active in the variety. In either case, male-fertility can be restored in the resulting plant by expression of the cytokinin biosynthesis enzyme(s).

In yet another aspect, the method further comprises transforming the plant cell comprising the construct encoding a cytokinin oxidase with an additional nucleic acid construct comprising an inducible promoter and a transcription termination sequence both operably linked to an antisense construct expression of which suppresses expression of the cytokinin oxidase. Alternatively, a male-sterile plant is produced by sexually crossing a plant comprising a nucleic acid construct containing a polynucleotide encoding a cytokinin oxidase with a plant, typically of the same variety, comprising a nucleic acid construct containing an inducible promoter and a transcription termination sequence both operably linked to an antisense construct expression of which suppresses expression of the cytokinin oxidase. In either case, male-fertility can be restored by expression of the antisense construct.

In another embodiment, the method further comprises transforming the plant cell comprising the construct encoding a cytokinin oxidase with an additional nucleic acid construct comprising an inducible promoter and a transcription termination sequence both operably linked to a polynucleotide encoding a dominant negative mutant protein expression of which decreases expression or activity of the cytokinin oxidase. Alternatively, a male-sterile plant is produced by sexually crossing a plant comprising a nucleic acid construct containing a polynucleotide encoding a cytokinin oxidase with a plant, typically of the same variety, comprising a nucleic acid construct containing an inducible promoter and a transcription termination sequence both operably linked to a polynucleotide encoding a dominant negative mutant protein, expression of which decreases expression or activity of the cytokinin oxidase. In either case, male-fertility can be restored by expression of the dominant negative mutant protein.

Additional aspects include seeds from plants produced by any of the preceding methods, uniform populations of plants produced by any of the preceding methods, and hybrid plants produced by crossing plants produced by any of the preceding methods with plants of a different variety.

Other features and aspects of the invention are described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying figures where:

FIGS. 1A-1B. (A) depicts T-DNA regions of pMON51826 and p MON51827. (B) depicts R0 transgenic plants of pMON51827 (S16400 and S16409) and pMON51826 (S16802, S16825, and S16832). All of the transgenic male sterile tassels shown in the figure are enlarged.

FIGS. 2A-2B depict genetic characterization of the transgenic lines. (A) DNA gel blot containing BamHI digests of genomic DNA isolated from leaves of lines S16400, S16409, S16802, S16825, S16832, and a wild type plant was hybridized with a NPTII probe. (B) The F1 plants resistant to kanamycin/paromomycin application and confirmed by a NPTII ELISA were scored as positives in the segregation analysis.

FIGS. 3A-3B detail expression analysis of CKX1 in transgenic plants. (A) The 70 kD CKX1 protein band appeared on the lanes containing transgenic leaves (youngest) and tassels at two stages (V10, V11) in the western blot. Sixty mg of leaf total protein and 20 mg of tassel total protein were loaded on the gel. (B) RT-PCR was performed on the total RNA isolated from leaves and tassels of transgenic and wild type plants. G3PDH was used as a positive cDNA control. CKX1mRNA was detected in the tassels as well as the young leaves (V11 stage) of transgenic plants. CKX1 mRNA was also present in the young leaves at the V8 stage. However, CKX1 transcripts were not detected in the mature leaves. The “no RT” controls consisted of a duplicated set of samples in which reverse transcriptase was not added in the cDNA synthesis step.

FIGS. 4A-4B delineate improved male organ development of F1 transgenic plants treated with kinetin and TDZ. (A) F1 transgenic plants sprayed with a mixture of kinetin and TDZ (+) or surfactant alone (−). From examples shown as lines S16802 and S16825, advanced floral development was visible on chemically treated plants whereas plants treated with surfactant only were devoid of floral development. (B) Although florets appeared to develop on the transgenic plants after kinetin and TDZ applications, the overall pollen production was insufficient for pollination. Pollen viability stains were then performed using Alexander's solution (Alexander, Stain Technol., 44:117-122, 1969). The chemically restored florets produced viable (purple and symmetrical in shape) pollen (panel I) similar to wild type. Some of the transgenic plants failed to rescue completely resulting in non-viable (blue and asymmetrical in shape) pollen (panel II).

FIGS. 5A-5B detail the potential use of CKX-generated, reversible male sterility in hybrid seed production. (A) When expressed in anthers, the CKX gene is a dominant male-sterility gene that can be maintained as a homozygous female parental line by chemical fertility restoration. In the hybrid seed production fields, hemizygous plants derived from a cross between homozygous transgenic and isogenic wild type plants are used as females to produce F1 hybrids with 50% of them being fertile. (B) When the CKX gene is specifically expressed in pollen, only the transgenic plants that are homozygous for the transgene are male sterile. These male sterile transgenic plants can be maintained by kinetin and TDZ applications. When used in production, hemizygous fertile F1 hybrids are produced.

FIG. 6 depicts the male sterile phenotype segregated with the transgene in F1 plants. Twenty-five F1 plants (S16832) were grown to maturity. The transgenic segregants identified by genomic DNA analysis and NPTII ELISA coincide with the male sterile phenotype.

ABBREVIATIONS AND DEFINITIONS

To facilitate understanding of the invention, a number of terms and abbreviations as used herein are defined below:

As used herein, the term “amino acid” is used in its broadest sense, and includes naturally occurring amino acids as well as non-naturally occurring amino acids, including amino acid analogs and derivatives. The latter includes molecules containing an amino acid moiety. One skilled in the art will recognize, in view of this broad definition, that reference herein to an amino acid includes, for example, naturally occurring proteogenic L-amino acids; D-amino acids; chemically modified amino acids such as amino acid analogs and derivatives; naturally occurring non-proteogenic amino acids such as norleucine, ornithine, etc.; and chemically synthesized compounds having properties known in the art to be characteristic of amino acids.

As used herein in reference to cytokinin oxidase, the term “biological activity” refers to the ability of the particular cytokinin oxidase to oxidatively remove cytokinin side chains to give adenine and the corresponding isopentenyl aldehyde. Also, as used herein, “cytokinin oxidase” and “cytokinin dehydrogenase” are used interchangeably.

As used herein, the terms “complementary” or “complementarity” refer to the pairing of bases, purines and pyrimidines, that associate through hydrogen bonding in double stranded nucleic acid. The following base pairs are complementary: guanine and cytosine; adenine and thymine; and adenine and uracil. The terms as used herein the terms include complete and partial complementarity.

As used herein, the term “hybridization” refers to a process in which a strand of nucleic acid joins with a complementary strand through base pairing. The conditions employed in the hybridization of two non-identical, but very similar, complementary nucleic acids varies with the degree of complementarity of the two strands and the length of the strands. Thus the term contemplates partial as well as complete hybridization. Such techniques and conditions are well known to practitioners in this field.

As used herein the term “linkage” refers to the situation in which two genes or segments of DNA are inherited together. Linked genes or DNA segments are located on the same chromosome and are generally located within 50 centiMorgans (cM) of each other. As used herein the term “linked” in reference to genes and DNA segments means that the genes and/or segments are located on the same chromosome within 50 cM, typically 10 cm, more typically 2 cm and more typically still 0.1 cm of each other.

As used herein, “peptide” and “protein” are used interchangeably and mean a compound that consists of two or more amino acids that are linked by means of peptide bonds.

As used herein, “polynucleotide” and “oligonucleotide” are used interchangeably and mean a polymer of at least 2 nucleotides joined together by phosphodiester bonds and may consist of either ribonucleotides or deoxyribonucleotides.

As used herein, a “recombinant nucleic acid” is defined either by its method of production or its structure. In reference to its method of production, e.g., a product made by a process, the process is use of recombinant nucleic acid techniques, e.g., involving human intervention in the nucleotide sequence, typically selection or production. Alternatively, it can be a nucleic acid made by generating a sequence comprising fusion of two fragments which are not naturally contiguous to each other, but is meant to exclude products of nature, e.g., naturally occurring mutants. Thus, for example, products made by transforming cells with any unnaturally occurring vector is encompassed, as are nucleic acids comprising sequences derived using any synthetic oligonucleotide process. Such is often done to replace a codon with a redundant codon encoding the same or a conservative amino acid, while typically introducing or removing a sequence recognition site. Alternatively, it is performed to join together nucleic acid segments of desired functions to generate a single genetic entity comprising a desired combination of functions not found in the commonly available natural forms. Restriction enzyme recognition sites are often the target of such artificial manipulations, but other site specific targets, e.g., promoters, DNA replication sites, regulation sequences, control sequences, or other useful features may be incorporated by design.

As used herein “recombinant protein” means that the protein, whether comprising a native or mutant primary amino acid sequence, is obtained by expression of a gene carried by a recombinant DNA molecule in a cell other than the cell in which that gene and/or protein is naturally found. In other words, the gene is heterologous to the host in which it is expressed. It should be noted that any alteration of a gene, including the addition of a polynucleotide encoding an affinity purification moiety to the gene, makes that gene unnatural for the purposes of this definition, and thus that gene cannot be “naturally” found in any cell.

As used herein “SDS” means sodium dodecyl sulfate.

As used herein, “secretion sequence” or “signal peptide or signal sequence” means a sequence that directs newly synthesized secretory or membrane proteins to and through membranes of the endoplasmic reticulum, or from the cytoplasm to the periplasm across the inner membrane of bacteria, or from the matrix of mitochondria into the inner space, or from the stroma of chloroplasts into the thylakoid. Fusion of such a sequence to a gene that is to be expressed in a heterologous host ensures secretion of the recombinant protein from the host cell.

As used herein, “sequence” means the linear order in which monomers occur in a polymer, for example, the order of amino acids in a polypeptide or the order of nucleotides in a polynucleotide.

As used herein “1×SSC” means 0.015 M NaCl/0.0015 M sodium citrate.

As used herein “1×SSPE” means 1.8 M NaCl, 10 mM NaH₂PO₄, 1 mM EDTA.

As used herein, “targeting sequence” means in the context of gene or polynucleotide insertion, a sequence which results in the gene or polynucleotide being inserted at a particular location by homologous recombination. In the context of proteins or peptides, “targeting sequence” refers to a nucleotide sequence encoding an amino acid sequence the presence of which results in a protein being directed to a particular destination within a cell.

As used herein, the term “transgenic” refers to a plant whose germline contains a gene or nucleic acid construct introduced using recombinant DNA technology. As used herein the term “transgene” means a gene or DNA construct which has been introduced into the germ line of a plant using recombinant DNA technology.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for producing plants with reversible male sterility has been discovered. These plants are produced by introducing polynucleotide constructs encoding a cytokinin oxidase, such as cytokinin oxidase 1 (CKX1). Male sterility in the transformed plants is due to expression and accumulation of cytokinin oxidase, which leads to a decrease in levels of cytokinin present in the plant. Because cytokinin is needed for normal pollen formation and male organ development, an increase in cytokinin oxidative degradation results in either a failure of normal pollen formation or a failure in male organ development. Male fertility can be restored by the restoration of normal cytokinin levels either by the application of natural or synthetic cytokinins, application of cytokinin oxidase inhibitors, inhibition of expression or activity of the introduced cytokinin oxidase, or any combination thereof.

Production of Transgenic Plants

Generally speaking, transgenic plants comprising cells containing isolated polynucleotides, vectors or expression cassettes of the present invention can be produced by transforming plant cells with a DNA construct as described herein via any of the foregoing methods; selecting plant cells that have been transformed on a selective medium; regenerating plant cells that have been transformed to produce differentiated plants; and selecting transformed plants that are male-sterile.

Any nucleic acid sequence encoding a cytokinin oxidase enzyme biologically active in the plant of interest can be used. The sequence used can be the coding region for the enzyme or can be a mutated and/or truncated form of the enzyme as long as the mutated and/or truncated form retains cytokinin oxidase activity. Polynucleotide sequences encoding cytokinin oxidase are known in the art and can be found, for example, in the GenBank data base maintained by the National Biotechnology Information Center available to the public at www.ncbi.nlm.nih.gov/ and the data bases maintained at the European Bioinformatics Institute and available to the public at www.ebi.ac.uk/. In addition, sequence data for cytokinin oxidase has been published by Morris et al. (Biochim. Biophys. Res. Commun., 255:328-333) and Houba-Herin et al. (Plant J., 17:615-626, 1999). In one embodiment, the polynucleotide encoding a Zea maize cytokinin oxidase 1 enzyme found in GenBank sequence accession no. AF044603 (SEQ ID NO: 1) may be used. In another embodiment, one of several polynucleotides encoding a Glycine max cytokinin oxidase found in GenBank sequence accession nos. BU926889 (SEQ ID NO: 13), BU084470 (SEQ ID NO: 14), BM528151 (SEQ ID NO: 15), BE330968 (SEQ ID NO: 16), AW830743 (SEQ ID NO: 17), BM143498 (SEQ ID NO: 18) or BG651837 (SEQ ID NO: 19) may be employed. In yet another embodiment, one of several polynucleotides encoding an Arabidopsis thaliana cytokinin oxidase found in GenBank sequence accession nos. AF303978(SEQ ID NO: 8), AF303979(SEQ ID NO: 9), AF303980(SEQ ID NO: 10), AF303981(SEQ ID NO: 11), AF303982(SEQ ID NO: 12), BT000179 (SEQ ID NO: 20), or AY091158 (SEQ ID NO: 21), may be employed. In still another embodiment, a polynucleotide encoding a Hordeum vulgare cytokinin oxidase found in GenBank sequence accession nos. AF540382 (SEQ ID NO: 22), or AF362472 (SEQ ID NO: 23) may be used. In yet a further embodiment, a polynucleotide encoding a Triticum aestivum cytokinin oxidase found in GenBank accession no. AF362471 (SEQ ID NO: 24) may be utilized.

Also included in the invention are polynucleotides that exhibit 90%, typically 92%, more typically 95%, and more typically, 98% sequence identity with SEQ ID NO: 1 or any of SEQ ID Nos: 8-24, or any of their complementary sequences. Such nucleotide sequences generally hybridize to the nucleic acid of SEQ ID NO: 1 or any of SEQ ID Nos: 8-24, or any of their complementary sequences under high stringency conditions. Generally speaking, hybridization procedures typically include initial hybridzation in 5×SSPE, 1-5× Denhardt's solution, 10-200 micrograms/ml denatured heterologous DNA, 0.5% SDS, at 50-68° C. for a time sufficient to permit hybridization (e.g. several hours to overnight) followed by two washes in 2×SSPE, 0.1% SDS at room temperature and two additional 15 minute washes in 0.1×SSPE, 0.1% SDS at 42° C. followed by detection of the hybridization products. Higher stringency washing can accomplished by at least one additional wash in 0.1% SSPE, 0.1% SDS at 55° C., more typically at 60° C., and more typically still at 65° C. High stringency hybridizations can also be carried out in 5×SSPE and 50% formamide at 42° C. followed by washing as described above (Meinkoth and Wahl, Anal. Biochem, 138:267-284 (1984)). As is well known by those of ordinary skill in the art, SSC can be substituted for SSPE in the above examples so that, for instance, hybridization can be accomplished in 5×SSC in place of 5×SSPE. It is well known to those of ordinary skill in the art that different compositions can result in equal stringency conditions for hybridization depending on well known factors such as the concentration of Na⁺, the percentage formamide, the temperature, the T_(m) of the hybrid to be formed, and the composition of the hybrid, e.g. DNA-DNA, DNA-RNA, or RNA-RNA. Thus the invention also encompasses nucleotide sequence that hybridize under conditions equivalent to those described above.

The polynucleotide sequence used can also encode either a secreted or non-secreted form of cytokinin oxidase. Non-secreted forms of CKX can be produced from secreted forms by the removal of the part of the sequence encoding the transit peptide necessary for secretion. Alternatively, polynucleotides encoding CKX polypeptides that are not naturally secreted such as AtCKX5 (Bilyeu et al., Pl. Physiol., 125:378-386, 2001) can be used.

Nucleic acid constructs of the present invention can be part of vectors and in particular, expression vectors or expression cassettes. In plants, transformation vectors capable of introducing polynucleotides encoding a cytokinin oxidase are easily designed, and generally contain one or more DNA coding sequences of interest under the transcriptional control of 5′ and 3′ regulatory sequences. Such vectors generally comprise, operatively linked in sequence in the 5′ to 3′ direction, a promoter sequence that directs the transcription of a downstream heterologous structural DNA in a plant; optionally, a 5′ non-translated leader sequence; a nucleotide sequence that encodes a protein of interest; and a 3′ non-translated region that encodes a polyadenylation signal which functions in plant cells to cause the termination of transcription and the addition of polyadenylate nucleotides to the 3′ end of the mRNA encoding the protein. Plant transformation vectors also generally contain a selectable marker. Typical 5′ to 3′ regulatory sequences include a transcription initiation start site, a ribosome binding site, an RNA processing signal, a transcription termination site, and/or a polyadenylation signal. Vectors for plant transformation have been reviewed in Rodriguez et al. (1988) Vectors: A Survey of Molecular Cloning Vectors and Their Uses, Butterworths, Boston; Glick et al. (1993) Methods in Plant Molecular Biology and Biotechnology CRC Press, Boca Raton, Fla; and Croy (1993) In Plant Molecular Biology Labfax, Hames and Rickwood, Eds., BIOS Scientific Publishers Limited, Oxford, UK. Non-limiting examples of plant transformation vectors useful in the present invention include pMON51826 and pMON51827, shown in FIG. 1.

Promoters useful in the present invention include those that confer appropriate cellular and temporal specificity of expression. Such promoters include those that are constitutive, inducible, environmentally-regulated, developmentally-regulated, organelle-specific, cell-specific, tissue-specific, male specific, anther-specific, pollen-specific, stamen-specific, tapetum-specific or any combination of the preceding, for example an inducible, male-specific promoter. Such promoters are well known to those of ordinary skill in the art. An example of a male-specific regulatory region can be found in U.S. Pat. No. 6,037,523. Examples of anther-specific promoters and regulatory sequences are disclosed U.S. Pat. Nos. 6,072,102; 5,962,769; 5,837,851; 5,795,753; 5,639,948; 5,589,610; 5,477,002; 6,013,859; 5,659,124; and 5,824,542. Examples of pollen-specific promoters are disclosed in U.S. Pat. Nos. 5,412,085; 5,086,169; 5,545,546; 6,018,104; 5,977,433; 5,545,546; and 5,086,169. Additional disclosure of pollen-specific promoters can be found in MacCormick (Trends Genet., 2:298-303, 1991), Hamilton et al. (Sex. Plant Reprod., 2:208, 1989) Guerrero et al. (Molec. Gen. Genet., 224:161-168, 1990), Weterings et al. (Plant Molec. Biol., 18:1101-1111, 1992) and Shen et al. (Molec., Gen. Genet., 234:379-389, 1992). Examples of tapetum-specific promoters and regulatory sequences can be found in U.S. Pat. Nos. 5,470,359; 5,639,948; 5,589,610; and European Patent No. 0,344,029. Further examples of tapetum-specific promoters and regulatory sequences can be found in Koltunow et al. (Plant Cell, 2:1201-1224, 1990), Hird et al. (Plant J., 4:1023-1033, 1993), Paul et al. (Plant Molec. Biol., 19:611-622, 1992), and Seurinck et al. (Nucl. Acids Res., 18:3403, 1990). Examples of stamen-specific promoters can be found in U.S. Pat. Nos. 5,639,948; 5,589,610; 5,880,331; 6,025,546; European Patent No. 0,344,029; and PCT Publication Nos. WO92/13956 and WO92/13957. In one embodiment, the pollen-specific promoter pZmg13 (Hanson et al., Plant Cell, 1:173-170, 1989) is used. In another embodiment, the anther-specific promoter pZtap (PCT Publication WO99/46396) is used.

A variety of different methods can be employed to introduce polynucleotides into plant protoplasts, cells, callus tissue, leaf discs, meristems, etc., to generate transgenic plants. These methods include, for example, Agrobacterium-mediated transformation, particle gun delivery, microinjection, electroporation, polyethylene glycol-mediated protoplast transformation, liposome-mediated transformation, etc. (Potrykus Annu. Rev. Plant Physiol. Plant Mol. Biol. 42: 205, 1991; Maliga et al., Methods in Plant Molecular Biology, Cold Spring Laboratory Press, 1995).

One aspect of the invention encompasses introduction of the polynucleotide into a dicot. Specific methods for transforming a wide variety of dicots and obtaining transgenic plants are well documented in the literature (Gasser and Fraley, Science 244:1293, 1989; Fisk and Dandekar, Scientia Horticulturae 55:5, 1993; Christou, Agro Food Industry Hi Tech, p. 17, 1994;and the references cited therein).

Another aspect of the invention encompasses introduction of the polynucleotide into a monocot. Successful transformation and plant regeneration have been achieved in the monocots as follows: asparagus (Asparagus officinalis; Bytebier et al. (1987) Proc. Natl. Acad. Sci. USA 84: 5345); barley (Hordeum vulgarae; Wan and Lemaux (1994) Plant Physiol. 104: 37); maize (Zea mays; Rhodes et al. (1988) Science 240: 204; Gordon-Kamm et al. (1990) Plant Cell 2: 603; Fromm et al. (1990) Bio/Technology 8: 833; Koziel et al. (1993) Bio/Technology 11: 194); oats (Avena sativa; Somers et al. (1992) Bio/Technology 10: 1589); orchardgrass (Dactylis glomerata; Horn et al. (1988) Plant Cell Rep. 7: 469); rice (Oryza sativa, including indica and japonica varieties; Toriyama et al. (1988) Bio/Technology 6: 10; Zhang et al. (1988) Plant Cell Rep. 7: 379; Luo and Wu (1988) Plant Mol. Biol. Rep. 6: 165; Zhang and Wu (1988) Theor. Appl. Genet. 76: 835; Christou et al. (1991) Bio/Technology 9: 957); rye (Secale cereale; De la Pena et al. (1987) Nature 325: 274); sorghum (Sorghum bicolor; Cassas et al. (1993) Proc. Natl. Acad. Sci. USA 90: 11212); sugar cane (Saccharum spp.; Bower and Birch (1992) Plant J. 2: 409); tall fescue (Festuca arundinacea; Wang et al. (1992) Bio/Technology 10: 691); turfgrass (Agrostis palustris; Zhong et al. (1993) Plant Cell Rep. 13: 1); and wheat (Triticum aestivum; Vasil et al. (1992) Bio/Technology 10: 667; Weeks et al. (1993) Plant Physiol. 102: 1077; Becker et al. (1994) Plant J. 5: 299).

Restoration of Male Fertility

Fertility can be restored in male-sterile lines by the restoration of normal cytokinin levels. By normal cytokinin levels is meant those levels of cytokinin that would be found in plants that are identical in their genetic make-up except for the presence of the exogenous recombinant oxidase gene. Typically, any means that restores cytokinin levels in a particular plant may be utilized. By way of example, suitable methods for the restoration of normal cytokinin levels can be achieved by application of cytokinins, cytokinin oxidase inhibitors or a combination of cytokinins and cytokinin oxidase inhibitors.

Generally speaking, cytokinins used to restore fertility can be either naturally occurring or synthetic. Synthetic cytokinins include those made by recombinant DNA technology as well as those made by conventional chemical synthesis. Examples of suitable native or synthetic cytokinins include kinetin, zeatin, zeatin riboside, dihydrozeatin, trans-zeatin, cis-zeatin, trans-zeatin riboside, isopentenyladenine, isopentenyladenosine and benzyladenine. Cytokinins can be conveniently applied to large numbers of plants in the form of a spray. In addition to the cytokinin and a suitable diluent, the spray can also contain other ingredients such as a surfactant to increase uptake of the cytokinin. Cytokinin can be used in a single application or multiple applications can be used depending one the situation. Typically, the cytokinin is applied for the duration of time necessary to restore the desired degree of male fertility in the particular plant species. Of course, the duration of its application will vary considerably depending upon the cytokinin employed and its rate of application, the particular plant species and its stage of development, and the desired degree of fertility restoration. One skilled in the art can readily determine the appropriate amount of time to apply the cytokinin. Moreover, the cytokinin can be applied at the V4, V7 and/or the V10 stages of development (See, Ritchie et al., How a Corn Plant Develops, Herman, ed., Iowa State University Cooperative Extension Service, Ames, Iowa, 1997). In one embodiment, the synthetic cytokinin, kinetin, is applied at a rate ranging between about 1 mg/plant to about 200 mg/plant, typically between about 3 mg/plant to about 100 mg/plant to restore male fertility.

Alternatively, a cytokinin oxidase inhibitor such as a cytokinin oxidase 1 inhibitor, can be used to restore normal cytokinin levels by inhibiting the increased breakdown of cytokinin due to the presence of the recombinant cytokinin oxidase 1 gene. As with the use of cytokinin to restore fertility, the cytokinin oxidase inhibitor used can be a naturally occurring or synthetic inhibitor. In one embodiment, the cytokinin oxidase inhibitor is an urea derivative. Examples of suitable urea derivatives include diphenyl and phenyl-pyridyl ureas, thiazole and thiazinoyl ureas, trisubstituted ureas, and thioureas. In another embodiment, the cytokinin oxidase inhibitor is a substituted cytokinin homolog. Generally speaking, suitable substituted cytokinin homologs include those cytokinins that have the N-6 hydrogens blocked with a methyl, fluoro, trifluoromethyl, or any other alkyl or aryl substituent. In still another embodiment, the cytokinin oxidase inhibitor comprises thidiazuron (TDZ) (Hare et al., Plant Cell Physiol., 35:1121-1125, 1994; Bilyeu et al., Zea Mays Plant Physiol., 125:378-386, 2001). The cytokinin oxidase inhibitor may conveniently be applied as a spray, which in addition to a diluent, may contain other useful ingredients such as a surfactant to improve uptake. Moreover, the inhibitor can be applied in a single application or in multiple applications and can be applied at the V4, V7 and/or V10 stages of development. Like the application of the cytokinin, the cytokinin oxidase inhibitor is typically applied for the duration of time necessary to restore the desired degree of male fertility in the particular plant species. Of course, the duration of its application will vary considerably depending upon the cytokinin oxidase inhibitor employed and its rate of application, the particular plant species and its stage of development, and the desired degree of fertility restoration. One skilled in the art can readily determine the appropriate amount of time to apply the cytokinin oxidase inhibitor. By way of example, when the cytokinin oxidase inhibitor is TDZ, it is typically applied as a spray at a rate ranging from between about 1 mg/plant to about 50 mg/plant, more typically from between about 3 mg/plant to about 30 mg/plant.

In another embodiment, male fertility is restored by applying a combination of cytokinin and cytokinin oxidase inhibitor. For example, any combination of kinetin and TDZ within the ranges given for each above can be used. It is also contemplated that a cytokinin having cytokinin oxidase activity may also be employed to restore male fertility. As with application of either cytokinins or cytokinin oxidase inhibitors alone, combinations can be applied as single or multiple applications and at the V4, V7 and/or V10 stages of development.

Male fertility can also be restored by the use of a second recombinant construct (restorer construct) that inhibits or interferes with expression of the construct encoding the cytokinin oxidase. In one embodiment, the restorer construct is operably linked to an inducible promoter so that application of the proper stimulus to drive expression of the restorer construct can be used to restore male fertility.

In one embodiment, the restorer construct encodes an antisense sequence. Antisense sequences can be produced by reversing the orientation of the transcribed region of a gene or polynucleotide sequence whose suppression is desired. When operatively coupled to a suitable inducible transcriptional promoter such as discussed herein, a transcript of the antisense DNA strand is produced when desired to restore fertility. The production and use of antisense DNA is well known in the art and can be found, for example, in Green et al., (1986) Annu. Rev. Biochem. 55:569. The transcript of the antisense DNA is antisense RNA. Without being bound by theory, it is believed that an individual antisense RNA molecule may hybridize with a complementary “sense” mRNA molecule to form an RNA-RNA duplex. Such a duplex may prevent the sense mRNA molecule from, for example, being translated or binding to another nucleic acid such as DNA. The presence of RNA-RNA duplexes may also initiate a sequence-specific RNA degradation pathway with the antisense molecules and/or the RNA-RNA duplexes playing a role in initiating the degradation pathway, and both sense and antisense molecules serving as specific targets for degradation. Waterhouse et al. (Proc. Natl. Acad. Sci. USA, 95:13959-13964, 1998) reported that the most efficient gene silencing was achieved in plants by simultaneous expression of sense and antisense RNA. Without being bound by theory, this efficiency is thought to be due to the formation of double stranded (ds) RNA. Thus the presence in plants of the present invention of sense and antisense constructs encoding cytokinin oxidase is thought to result in a high efficiency of cytokinin oxidase inhibition.

It is also contemplated that the antisense transcript need not encompass the entire sense sequence, but may be a fragment which hybridizes to only a portion of the sense RNA. The antisense transcript should be of sufficient length to allow specificity in binding to the target (sense) transcript. In general, the antisense transcript should be at least 10 bases long, more typically at least 20 bases long, although the presence of rare sequences may allow the use of shorter antisense transcripts. The use of this technology to suppress the expression of specific plant genes has been described, for example in European Patent Publication No 271988; U.S. Pat. Nos. 5.073,676, 5,107,065 and 5,569,831; PCT Publication WO 00/49035, Smith et al, (1988) Nature, 334, 724-726; and Smith et al, (1990) Plant Mol. Biol. 14, 369-380).

In yet another embodiment, male fertility may be achieved by the use of a restorer construct containing a dominant negative mutation. Expression of a restorer construct containing a dominant negative mutation generates a mutant transcript that, when coexpressed with the cytokinin oxidase, inhibits the action of the cytokinin oxidase transcript. Methods for the design and use of dominant negative constructs are well known in the art and can be found, for example, in Herskowitz, Nature, 329:219-222, 1987 and Lagna and Hemmati-Brivanlou, Curr. Topics Devel. Biol., 36:75-98, 1998.

In still another embodiment, the restorer construct encodes a ribozyme that cleaves the recombinant cytokinin oxidase transcript. Ribozymes are catalytic RNA molecules that can promote specific biochemical reactions without the need for auxiliary proteins. Reactions catalyzed by ribozymes can be either intramolecular or intermolecular. Examples of intramolecular reactions are self-splicing or self-cleaving reactions while intermolecular reactions involve other RNA molecules as substrates and more closely approximate true enzymatic reactions where the enzyme is unchanged after each reaction. In the present method, ribozymes catalyzing intermolecular reactions are generally employed. The use of ribozymes to alter expression of genes is discussed in Cech, J. Am. Med. Assoc., 260:3030-3034 (1988).

Uses of Reversible Male Sterility

Generally speaking, the method of the present invention may be advantageously employed as a means for generating and maintaining male sterility in any plant species and in hybrids of any species. The method not only reduces the expense of seed production for existing hybrid plants such as corn, but also makes it possible to produce hybrid varieties of traditionally non-hybrid crops.

One aspect of the invention, therefore, encompasses use of the method to produce hybrid varieties having reversible male sterility in plants where hybrid varieties are traditionally employed in seed production. In one alternative of this embodiment, the plant is corn. By way of example, FIG. 5A outlines one embodiment to generate reversible male sterility in the production of hybrid corn employing the method of the invention. The method detailed in this example utilizes CKX linked to an anther specific promoter. When expressed in the anthers, CKX acts as a dominant male-sterility gene. Homozygous male-sterile female lines can be maintained, for example, by the use of chemical fertility restoration as described herein. The homozygous male-sterile female line is crossed with isogenic wild type plants to produce a hemizygous male-sterile female line. In hybrid seed production fields, the hemizygous line is crossed with a male line to produce F1 hybrid seeds. When planted, 50% of the resulting plants will be male-sterile, however, in species such as corn which produce an excess of pollen, the remaining 50% of the plants that are male fertile will provide sufficient pollen for pollination and thus normal yields. Furthermore, it has been suggested that out-crossed male sterile corn plants have a yield advantage over fertile plants due to resource reallocation from abolishing the development of male tissues (Hunter et al., Agron. J., 65:471-472, 1973). Thus a hybrid population composed of 50% male steriles has the potential to deliver increased yields.

In yet another aspect, the method may be employed to produce hybrid varieties having reversible male sterility in plants where hybrid varieties are not traditionally utilized in seed production. Typical plants species where hybrid varieties are not employed in seed production include wheat, rice, canola and cotton. By way of example, FIG. 5B outlines one embodiment to generate reversible male sterility in the production of hybrid varieties in one of these plant species employing the method of the invention. In this example, expression of CKX is driven by a pollen-specific promoter, in which case only homozygous plants are male sterile. Homozygous male-sterile female lines are produced by selfing hemizygous lines and then the lines are maintained through restoration of male fertility, for example, by chemical restoration as described herein. The homozygous male-sterile female line is then crossed with a male line to produce F1 hybrid seed. This seed will result in fertile F1 hybrids with 50% sterile pollen.

In addition to the method for producing reversible male sterility in transgenic plants, plants produced by the method disclosed herein are also considered within the scope of the present invention. Such plants can be produced by the methods described above and contain a recombinant construct encoding a cytokinin oxidase. Optionally, the plant can further comprise a restorer construct such as those described above. Such plants can be either of the class dicotyledonae (dicots) or the class monocotyledonae (monocots). When first produced, the plant will be hemizygous for the introduced recombinant constructs. Such plants, however, can be made homozygous by “selfing” a technique which is well known in the art of plant breeding. Also within the scope of the present invention are uniform populations of plants which have been altered by the methods of the present invention such that they possess reversible male sterility.

Also included within the scope of the present invention are seeds and progeny from any of the above described plants. Another aspect of the present invention encompasses a method for the production of a hybrid plant comprising sexually crossing a plant having reversible male sterility and produced by the methods described herein with a plant of a different variety as well as the hybrid plants produced, uniform populations of such hybrid plants and seeds from such hybrid plants.

The present invention also provides a method for preventing or reducing cross-pollination of transgenic plants with wild type plants. With the increasing introduction of transgenic plants into production agriculture, there has been concern about the possible effects due to cross pollination between transgenic and wild type plants. Using the present methods, transgenic male sterile plants can be produced and planted together with a small number of non-transgenic plants to act as pollen donors. In this way, only wild type pollen would be released into the environment. By way of a non-limiting example, a transgenic hybrid plant combining insect resistance and an anther-expressed cytokinin oxidase, typically cytokinin oxidase 1, can be produced using the methods described herein. Interplanting with a small number of non-transgenic hybrids would provide a “refuge” for resistant management as well as pollen donors for complete pollination in the production field of the transgenic plants. Since the transgenic plants would be male-sterile, there would be no or greatly reduced pollination of wild type plants with transgenic pollen. As used herein in reference to cross-pollination “reduced” means that the incidence of pollination of wild type plants by transgenic pollen is less than occurs in the absence of an anther- or pollen-expressed cytokinin oxidase.

Similar results can be obtained using a plant which is hemizygous for a desirable transgene linked to a pollen-expressed cytokinin oxidase, typically a cytokinin oxidase 1. In this situation, only wild type pollen and not pollen producing the transgene linked to the expressed cytokinin oxidase would be produced. In one embodiment, the transgene and pollen expressed cytokinin oxidase are under the control of different regulatory sequences so that while the cytokinin oxidase is expressed primarily or typically only in pollen, the transgene is expressed in a wider variety of cell types. In this way, pollination of wild type plants with transgenic pollen can be avoided or reduced. Such plants can be produced by transforming plant cells with the linked transgene and pollen expressed cytokinin oxidase by any of the methods described herein, and regenerating a plant from said transformed cells. Alternatively, such plants can be produced by sexually crossing a plant homozygous for the linked transgene and pollen expressed cytokinin oxidase with a homozygous wild type (non-transgenic) plant to produce a hemizygous hybrid.

The present methods are also useful in the introduction of economically valuable traits from plants having, in general, undesirable production characteristics into plants having desirable characteristics (elite plants or varieties) and in particular elite hybrids. This method of introducing valuable traits is known as TopCrossing and is detailed in U.S. Pat. Nos. 5,704,160 and 5,706,603; both of which are herein incorporated by reference.

An example of the use of a TopCross can be found in corn. Most varieties of corn grown commercially are developed to meet common manufacturing and feeding requirements. There exists, however, a market for specialty varieties which differ from standard varieties by the presence of one or more economically important transgenic or non-transgenic traits. In corn, such traits include, but are not limited to, degree of starch branching, increased accumulation of sugars or water-soluble polysaccharides, degree of endosperm hardness, protein or amino acid content, and oil content. Unfortunately, many plants that possess these economically important traits also possess characteristics that make them agronomically undesirable. TopCross provides a means to circumvent this problem by capturing economically important traits from agronomically inferior plants through elite varieties. An elite variety, as is well known in the art, is a variety which possesses a favorable combination of traits making it particularly useful in commercial production.

In a TopCross, male-sterile plants of an elite variety are interplanted with a low density of male-fertile plants having the desired trait. By “low density” is meant that the male-fertile plants are interplanted at a rate which does not exceed about 10 male-fertile plants per 100 male-sterile plants. Due to low density planting, the presence of the agronomically inferior plants does not significantly reduce overall yields. The reversible male-sterility of the present invention is particularly useful in production of high value plants and grains by TopCross. For example, an agronomically elite variety, typically a hybrid variety, which has been made reversibly-male sterile by the methods of the present invention, can be randomly interplanted with an agronomically inferior variety which possesses a desirable trait and which is present in the field at a low density. The inferior variety is allowed to pollinate the elite variety allowing the economically important trait to be expressed in the plant and/or grain produced.

EXAMPLES

The following examples are intended to provide illustrations of the application of the present invention. The following examples are not intended to completely define or otherwise limit the scope of the invention.

Example 1 Vector Construction

The complete 1.7-kb corn cytokinin oxidase (CKX1)-containing DNA fragment including the transit peptide was inserted into expression cassettes driven by either the pollen-specific pZmg13 promoter (Hanson et al., Plant Cell, 1:173-179, 1989) or the anther-specific pZtap promoter (PCT Publ. No. WO 99/46396), and terminated by the nopline synthase sequence (NOS3′). These cassettes also included an intron from a heat shock protein (HSP70 intron) to increase the expression of CKX1. The resulting CKX1 expression cassettes were cloned into binary vectors using an enhanced CaMV 35S promoter (e35S)-driving NPTII as the selectable marker to generate pMON51826 and pMON51827 (FIG. 1A). All other genetic elements used in the binary vector were identical to those described by Ye et al (Plant J., 19:249-257, 1999).

Example 2 Transformation, Plant Material and Growth Conditions

The binary vectors, pMON51826 and pMON51827 were electroporated into Agrobacterium tumefaciens ABI strain and introduced into corn embryos (LH198 X Hill) by Agrobacterium-mediated transformation. H99 pollen was used to pollinate R0 male sterile transgenic plants to produce F1 seeds. All of the plants were grown in greenhouses at 28/21° C. (day/night) with a 16-hr photoperiod (400 mmol m⁻² sec⁻¹), and 50% relative humidity.

Thirty-three and 31 independent RO transgenic plants were generated harboring pMON51826 and pMON51827, respectively. Despite the difference in the promoters, 29 plants each of pMON51826 and pMON51827 displayed a similar complete, sporophytic male sterile phenotype. At the apex of the plants where the tassels ordinarily form, the male sterile transgenic plants produced rudimentary terminal structures that lacked recognizable male florets or spikelets (FIG. 1B). These transgenic plants were also shorter and had narrower leaves than wild-type plants. Pollen from wild-type plants successfully fertilized several male sterile transgenic plants to yield F1 seeds. However, about half of the independently transformed CKX1 events did not produce fertile ears when out-crossed.

Example 3 Plant DNA Isolation and F1 Segregation Analysis

Five independent lines were selected for genomic DNA analysis (FIG. 2A). Genomic DNA was isolated from leaves of corn plants using the DNeasy Plant Mini Kit (Qiagen, Valencia, Calif.). Genomic DNA (20 mg) was digested with restriction enzymes (BamHI), separated on a 0.8% (w/v) agarose gel, and transferred to positively charged nylon membranes (Roche Molecular Biochemicals, Indianapolis, Ind.). Prehybridization, hybridization, washing and detection of the membranes were conducted using the non-radioactive DIG system (Roche Molecular Biochemicals) following the manufacturer's protocols.

The presence of NPTII in the F1 plants was determined by both antibiotic resistance and the NPTII ELISA from leaf protein extracts. An antibiotic solution containing 1 g/L kanamycin, 1 g/L paromomycin and 0.6% (v/v) Silwet L77 surfactant (Loveland, Greenley, Colo.), was applied to plants at V2 or V3 stages. Three days after application, the kanamycin susceptible plants began to display chlorosis symptoms on their leaves. The NPTII Pathoscreen Kit was purchased from Agdia (Elkhart, Ind.) and used for NPTII ELISAs following the manufacturer's protocol.

Detection of a single copy of the transgene by DNA gel-blot analysis (FIG. 2A) in these five lines correlated with a 1:1 F1 segregation ratio of the selectable marker (FIG. 2B). These results also indicate that the female gamete development is normal in the transgenic plants. Most of these F1 plants were grown to full maturity and the male sterile phenotype was inherited by plants containing the transgene (FIG. 6). Table 1 details normal transgene transmission from the females of male sterile transgenic plants.

TABLE 1 Harbored Line Constructr^(a) (NPTII⁺/NPII⁻)^(b) X²(1:1) S16400 pMON51827 15/17 0.13 S16409 pMON51827 25/29 0.30 S16802 pMON51827 19/16 0.26 S16825 pMON51826 22/27 0.51 S16832 pMON51826 57/53 0.15 ^(a)CKX1 was driven by pZmg13 (pMON51826) or pZtap (pMON51827) promoter. ^(b)The presence or absence of the transgene in the F1 plants was determined by both the antibiotic resistance and the NPTII ELISA from leaf protein extracts. NPTII⁺, NPTII-positive; NPTII⁻, NPTII-negative.

Example 4 Western Blots

In addition to male sterility, the transgenic plants produced from both vectors also exhibited other phenotypic abnormalities. Since cytokinins are known to be involved in many processes of plant growth and development, it was speculated that the observed effects were due to expression of the recombinant construct in non-reproductive tissues despite the presence of male-specific promoters. To test this, protein samples were obtained from transgenic leaves (youngest) and tassels at two stages of development (V10 and V11). Protein samples were prepared in an extraction buffer containing 1× PBS, 1 protein inhibitor cocktail tablet/20ml (Roche Molecular Biochemicals, Indianapolis, Ind.), and 0.05% Tween 20 (v/v), and quantified by Bio-Rad Protein Assay (Hercules, Calif.). Twenty or 60 mg of total protein were separated on a 10-20% Ready Gel and blotted to a 0.2 mm PVDF membrane following the manufacturer's protocol (Bio-Rad). The blot was probed with rabbit anti-CKX1 polyclonal antibody (Bilyeu et al., Plant Physiol., 125:378-386, 2001) at a dilution of 1:10,000, and then incubated with horseradish peroxidase-labeled anti-rabbit antibody at a 1:5,000 dilution. The immunocomplexes were visualized by enhanced chemiluminescence (ECL) according to the instructions of the manufacturer (Amersham Pharmacia, Piscataway, N.J.). The results are shown in FIG. 3A. As shown in the figure, the presence of the CKX1 protein was detected in the leaves and tassels of transgenic plants at both the V10 and V11 stages.

Example 5 RT-PCR

To determine whether the presence of the CKX1 protein in transgenic leaves was due to leaky expression of the male-specific promoters or secretion of the CKX1 protein from male-specific tissues, RNA analysis was performed on the F1 transgenic plants of line S16832. CKX1 transcripts were not detected by total RNA blot analysis, so reverse transcriptase-polymerase chain reaction (RT-PCR) was used. Total RNA was isolated at various stages from leaves and tassels of corn plants by using TRIzol Reagent (Life Technologies, Gaithersburg, Md.) followed by first strand cDNA synthesis (SuperScript™ Preamplication System, Life Technologies) with 2 mg of RNA each in a 20 ml reaction. One ml of the resulting cDNA mix was used in a 30 ml PCR solution containing 1× Reaction Buffer, 15 mM MgCl₂, 0.2 mM dNTPs, 6% (v/v) DMSO, 0.2 mM of each primer, and 0.5 unit of Taq DNA Polymerase. The reaction was amplified for 30 cycles. Each cycle consisted of denaturation at 92° C. for 1 min, annealing at 55° C. for 1 min, and extension at 72° C. for 1 min, after an initial denaturation for 2 min. Immediately following the last cycle, the samples were left at 72° C. for 5 min to complete extension. The gene encoding glyceraldehyde-3-phosphate dehydrogenase (G3PDH) (Russell et al., Plant Cell, 1:793-803, 1989) was used as a positive control. The sequences of the primers used to amplify NPTII, G3PDH, and CKX1 and predicted product sizes are listed in Table 2. All chemicals and enzymes were from Sigma (St. Louis, Mo.) unless otherwise noted.

TABLE 2 PRODUCT SEQ ID SIZE GENE PRIMERS NO. (bp) NPTII 5′-CGCTTGGGTGGAGAGGCTATTC-3′ 2 741 5′-GAAGGCGATAGAAGGCGATGCG-3′ 3 G3PDH 5′-CCCCATGTTCGTTGTTG-3′ 4 576 5′-TATCCCCACTCGTTGTCGTACC-3′ 5 CKX1 5′-CGGCACGCTGTCCAACGC-3′ 6 362 5′-GGCTCTGGTTCACGAACACC-3′ 7

As shown in FIG. 3B, in addition to tassels, the CKX1 mRNA was detected in the young leaves of transgenic plants at V11 stage, where the CKX1 protein accumulated. The CKX1 mRNA was also present in the young leaves at V8, a stage prior to tassel development in transgenic plants. However, unlike neomycin phosphotransferase type II (NPTII) which was driven by a constitutive promoter, enhanced CaMV 35S (e35S) (Fang et al., Plant Cell, 1:141-150, 1989), the CKX1 transcripts were not detectable in the mature leaves. The detection of CKX1 expression in non-male tissues by RT-PCR was not due to contamination by genomic DNA, since the negative controls that lack reverse transcriptase were absent of any visible PCR products.

Example 6 Fertility Restoration

Two commercially registered chemicals were tested in fertility restoration experiments. Kinetin is a synthetic cytokinin that is not oxidized by CKX1 and TDZ is a known cytokinin oxidase inhibitor. The F1 seeds from the crosses between wild-type plants and several male-sterile R0 events were planted. NPTII ELISA positive plants were identified as the male-sterile transgenic plants among F1 segregating populations by enzyme-linked immunosorbent assays (ELISA, Example 3) performed on leaf samples collected at the seedling stage of all plants. Identified plants were treated at the V4, V7 and V10 stages. The application rates for each plant were 0, 10, 20, 30, and 100 mg of kinetin with or without 3 mg of TDZ in the presence of 0.25% of Sylgard 309 surfactant (Willbur-Ellis, Fresno, Calif.). One to three plants were tested at a particular rate combination.

It had previously been shown that these lines had a single transgene insert and that the NTPII ELISA positive phenotype cosegregated with the male sterile phenotype. The NPTII ELISA tests were thus used to select male sterile F1 transgenic plants for chemical treatments. In several instances, as shown in FIG. 4A, the tassels of chemically treated transgenic plants were more developed than controls treated with surfactant only. These chemical application conditions were not sufficient to produce fully fertile plants. The degree of floral development restoration varied and was limited to older tissues of the tassel. In these treated transgenic plants, the earlier flowers produced viable pollen (FIG. 4B, panel I), but the later flowers did not (FIG. 4B, panel II) even though filaments and anthers had developed. Table 3 details tassel fertility rating of the restoration study.

TABLE 3 TDZ and kinetin TDZ and kinetin applications at V4 + V7 + V10 applications at V7 Line 0, 0^(a) 0, 20 3, 20 0, 100 3, 100 16802 0 0 xx 0 x 16825 0 0 xx 0 x 16832 0 0 xx 0 0 16400 0 0 0 0 x 16409 0 0 0 x ^(a)“#, #” indicate mg/plant of TDZ and kinetin in each treatment. Two to three transgenic F1 plants were used in each treatment group. Plants were either treated repeatedly in three different rates at three V4, V7 and V10 developmental stages or once in a higher rate at V7 developmental stage. Rates are indicated by mg/plant of TDZ and kinetin, respectively, in the presence of 0.25% of Sylgard 309 surfactant. The fertility rating was based on the visual observation. 0, No or very little floral development; x, Advanced floral development observed in some areas of tassels; xx, Increased areas of advanced floral development and anthersis observed in some flowers.

The observation of the aborted pollen in the later flowers supports previous evidence of the requirement of cytokinins in pollen development. Also, on the same tassels that the restored flowers were observed, sectors that lacked floral development remained.

In light of the detailed description of the invention and the examples presented above, it can be appreciated that the several aspects of the invention are achieved.

It is to be understood that the present invention has been described in detail by way of illustration and example in order to acquaint others skilled in the art with the invention, its principles, and its practical application. Particular formulations and processes of the present invention are not limited to the descriptions of the specific embodiments presented, but rather the descriptions and examples should be viewed in terms of the claims that follow and their equivalents. While some of the examples and descriptions above include some conclusions about the way the invention may function, the inventors do not intend to be bound by those conclusions and functions, but put them forth only as possible explanations.

It is to be further understood that the specific embodiments of the present invention as set forth are not intended as being exhaustive or limiting of the invention, and that many alternatives, modifications, and variations will be apparent to those of ordinary skill in the art in light of the foregoing examples and detailed description. Accordingly, this invention is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the following claims. 

1. A method for producing a plant characterized by reversible male-sterility, the method comprising: (a) transforming a plant cell with a first nucleic acid construct containing a polynucleotide encoding a cytokinin oxidase; (b) transforming the plant cell with a second nucleic acid construct comprising an inducible promoter and a transcription termination sequence both operably linked to a polynucleotide encoding at least one cytokinin biosynthesis enzyme; (c) regenerating a plant from the plant cell wherein expression of the cytokinin oxidase inhibits pollen formation or male organ development in the plant; and (d) restoring male-fertility by restoring normal cytokinin levels in the plant.
 2. The method of claim 1 wherein the first construct further comprises a regulatory sequence and a transcription termination sequence both operably linked to the polynucleotide encoding the cytokinin oxidase.
 3. The method of claim 2 wherein the regulatory sequence comprises a promoter.
 4. The method of claim 3 wherein the promoter is selected from the group consisting of constitutive, inducible, environmentally-regulated, developmentally-regulated, organelle-specific, cell-specific, tissue-specific, male specific, anther-specific, pollen-specific, stamen-specific, tapetum-specific promoters and any combination thereof.
 5. The method of claim 3 wherein the promoter is a pollen-specific promoter.
 6. The method of claim 3 wherein the promoter is an anther-specific promoter.
 7. The method of claim 1 further comprising sexually crossing the plant comprising the polynucleotide encoding a cytokinin oxidase with a plant of the same variety comprising a nucleic acid construct containing an inducible promoter and a transcription termination sequence both operably linked to a polynucleotide encoding at least one cytokinin biosynthesis enzyme.
 8. The method of claim 1 wherein male-fertility is restored by expression of the polynucleotide encoding at least one cytokinin biosynthesis enzyme.
 9. The method of claim 1 further comprising selfing the plant to produce a plant homozygous for the polynucleotide encoding a cytokinin oxidase.
 10. A seed from a plant produced by the method of claim 1, wherein said seed comprises the first nucleic acid construct containing a polynucleotide encoding a cytokinin oxidase and the second nucleic acid construct containing a polynucleotide encoding at least one cytokinin biosynthesis enzyme.
 11. A uniform population of plants produced by the method of claim
 1. 12. A method of producing a hybrid plant comprising sexually crossing a plant produced by the method of claim 1 with a plant of a different variety.
 13. A hybrid plant produced by the method of claim 12, wherein said plant comprises the first nucleic acid construct containing a polynucleotide encoding a cytokinin oxidase and the second nucleic acid construct containing a polynucleotide encoding at least one cytokinin biosynthesis enzyme.
 14. A seed produced from the plant of claim 13, wherein said seed comprises the first nucleic acid construct containing a polynucleotide encoding a cytokinin oxidase and the second nucleic acid construct containing a polynucleotide encoding at least one cytokinin biosynthesis enzyme.
 15. The method of claim 1 wherein the plant is a monocot.
 16. The method of claim 15 wherein the monocot is Zea mays.
 17. The method of claim 1 wherein the cytokinin oxidase comprises cytokinin oxidase
 1. 18. The method of claim 1 wherein the polynucleotide encoding cytokinin oxidase is selected from the group consisting of: (a) a polynucleotide comprising SEQ ID NO: 1 or the full-length complement thereof; (b) a polynucleotide that has at least 90% sequence identity with the polynucleotide of (a); (c) a polynucleotide that hybridizes to the polynucleotide of (a) under conditions of 5×SSC, 50% formamide and 42° C., and which encodes a protein having the same biological function; (d) a polynucleotide encoding the same amino acid sequence as (a), but which exhibits regular degeneracy in accordance with the degeneracy of the genetic code; (e) a polynucleotide encoding the same amino acid sequence as (b), but which exhibits regular degeneracy in accordance with the degeneracy of the genetic code; and (f) a polynucleotide encoding the same amino acid sequence as (c), but which exhibits regular degeneracy in accordance with the degeneracy of the genetic code. 